On the origin of the intracluster light in massive galaxy clusters

Abstract

We present a pilot study on the origin and assembly history of the intracluster starlight (ICL) for four galaxy clusters at 0.44 ≤ z ≤ 0.57 observed with the Hubble Space Telescope from the Cluster Lensing and Supernova Survey with Hubble (CLASH) sample. Using this sample of CLASH clusters we set an empirical limit on the amount of scatter in ICL surface brightness profiles of such clusters at z = 0.5, a mean of 0.24 mag arcsec−2 for 10 < r < 110 kpc, and constrain the progenitor population and formation mechanism of the ICL by measuring the ICL surface brightness profile, the ICL colour and colour gradient, and the total ICL luminosity within the same radial range. This scatter is physical – it exceeds the observational errors, straightforward expectations from the range of cluster masses in our sample, and predictions based on published evolutionary models for the variance attributable to the redshift span of our sample. We associate the additional scatter with differences in ICL assembly process, formation epoch, and/or ICL content. Using stellar population synthesis models we transform the observed colours to metallicity. For three of the four clusters we find clear negative gradients that, on average, decrease from supersolar in the central regions of the brightest cluster galaxy (BCG) to subsolar in the ICL, under the assumption that the age of the intracluster stars is >11 Gyr. Such negative colour (and equivalently, metallicity) gradients can arise from tidal stripping of L* galaxies and/or the disruption of dwarf galaxies, but not major mergers with the BCG. We also find that the ICL at 110 kpc has a colour comparable to m* + 2 red sequence galaxies, suggesting that out to this radius the ICL is dominated by stars liberated from galaxies with L > 0.2 L*. Finally, we find ICL luminosities of 4–8 L* in the range 10 < r < 110 kpc for these clusters. Neither dwarf disruption nor major mergers with the BCG alone can explain this level of luminosity and remain consistent with either the observed evolution in the faint-end slope of the luminosity function or predictions for the number of BCG major mergers since z = 1. Taken together, the results of this pilot study are suggestive of a formation history for these clusters in which the ICL is built-up by the stripping of >0.2L* galaxies, and disfavour significant contribution to the ICL by dwarf disruption or major mergers with the BCG.